If you say
that something will happen if necessary
, when necessary
, or where necessary
, you mean
that it will happen if it is necessary, when it is necessary, or where it is necessary.
If necessary, the airship can stay up there for days to keep out of danger.
The army needs men who are willing to fight, when necessary.
All the rigging had been examined, and renewed where necessary.
COBUILD Advanced English Dictionary. Copyright © HarperCollins Publishers